The present invention relates to a method of making spars for helicopter blades.
Helicopter blades in use today are usually made of a synthetic material and comprise a suction side and a pressure side which are substantially symmetrical with respect to a chord of the blade. The suction side and the pressure side of a blade normally each include two axially adjacent substantially U-shape ribs each constituted by a tape of reinforced resin which is folded in such a way as to form two adjacent substantially rectilinear twisted arms joined together along a curved section. This latter, in the finished blade, surrounds one of the two holes for attachment of the root of the blade to the rooter hob.
The ribs mentioned above are normally made by means of an automatic positioning device which is made to move with a reciprocating movement along a U-shape path to deposit the said tape in contact with the straight sides of a preliminary mould. The rib, once roughly formed on this mould, is transferred to a first forming mould on which the desired twist is conferred to the arms of the rib.
Each rib is then transferred, together with another rib parallel to and alongside it, to the interior of a second forming mould in such a way as to define the suction side or the pressure side of a blade which is subsequently coupled to a pressure side or, respectively to a suction side to define the spar, that is to say the supporting skeleton of the blade itself.
The method desired above has indisputable advantages, the most important of which lies in the simplicity of the automatic positioning device for the tape. In fact, this device does not have to be capable of following the twisted surface of the first forming mould, but only the straight surface of the preliminary mould.
Notwithstanding the above, the known method previously described is not entirely devoid of disadvantages which are due, above all, to the complications involved in the use of several moulds, to the lack of precision normally introduced by the transfer of the tape from the preliminary mould to the forming mould, and by the adjustment to the first forming mould of the tape which has been laid up on the preliminary mould, which adjustment is often performed manually. Another disadvantage is the relatively long time taken for positioning the tape, due to the fact that the tapes are deposited on the preliminary mould with reciprocating displacements of the positioning device which necessarily involve relatively low average speeds.